Viscosity regulated cooling system

ABSTRACT

A viscosity regulated cooling system for cooling a load device comprises a closed coolant circulating system adapted to contain a coolant whose viscosity varies inversely with its temperature. the circulating system has coolant circulating means for supplying and circulating coolant through the closed system, refrigeration means for cooling the coolant, load means for bringing the coolant, after being cooled by the refrigeration means, into heat absorbing relationship with the load device, and viscosity-responsive flow restriction means immediately downstream of the load means and upstream of the circulating means for limiting the flow of the coolant responsive to its viscosity as established by the load means.

United States Patent Porter May 6, 1975 Primary Examiner-Meyer Perlin Attorney, Agent, or FirmW. R. Hulbert [75] Inventor: John H. Porter, Colchester Point,

57 ABSTRACT Assign: Research lndustriesv A viscosity regulated cooling system for cooling a load Burlmgtoni device comprises a closed coolant circulating system 2 Filed; May 24 7 adapted to contain a coolant whose viscosity varies inversely with its temperature. the circulating system has [21] PP 363564 coolant circulating means for supplying and circulating coolant through the closed system, refrigeration 52 0.5. Cl 62/216; 62/333 means for cooling the Coolant, load means for bringing 511 int. c1. F25d 17/00 the Coolant, after wing cooled y the refrigeration [58] Field of Search 62/216, 333, 79, 175, 335 means, into heat absorbing relationship with the load device, and viscosity-responsive flow restriction means [56] References Cited immediately downstream of the load means and up- UNITED STATES PATENTS stream of the circulating means for limiting the flow of 2 056 786 long Harbo dt 62,333 the coolant responsive to its viscosity as established by 2,224,629 l2ll940 Borse i 62/333 the load means 2,379,249 6/1945 Muskat 62/333 4 Claims, 2 Drawing Figures /COOLANT CIRCULATING SYSTEM (I21 Z I 34 32 ;O

COOLANT I RESERVOIR mTERoooLER RESTRICTOR I l I LOAD LOAD 24 26 I NEANS [EVlCE ---w C(IJLANT I x: PUMP L' AL ne ies L -4- l -l l HEATI P27 l M '0 I [EZ E EiffJ I I CAPILLARY l6 TUBING I I COMPRESSOR I CONDENSER I REFRIGERANT I l L a J REFRlGERATlON SYSTEM (I4) PATENTEDHAY'BIDYS 3 91.323

sum 1 or 2 /COOLANT CIRCULATING SYSTEM (:2)

COOLANT 28 I RESERVOIR D INTERCOOLER RESTRICTOR I LOAD LOAD MEANS DEVlCE 24 2 I .1 l I PUMP 'z z COOLANT I 3| i r a :HEAHY' H27 '0 I v E A OEA:I'-OR 1| CAPILLARY TUBING COMPRESSOR |7 CONDENSER REFRIGERANT I I8 J \REFRIGERATION SYSTEM (14) PATENTEIJIIAY ems SHEET 2 BF 2 FIG 2 PETROLEUM OIL SAE IOW3O ETHYLENE GLYCOL WATER III DOW CORNING 200 SILICONE TEMPERATURE F VISCOSITY REGULATED COOLING SYSTEM This invention relates to cooling systems. In particular. it relates to cooling systems for use with equipment such as diffusion pumps for use in vacuum evaporators. electron microscopes and other high vacuum equipment.

It is an object of the present invention to provide a self-regulating cooling system for maintaining a thermal load at an essentially constant temperature.

It is a further object to provide such a system that needs a refrigeration subsystem of smaller capacity. dimensions and weight relative to the thermal load. than has hitherto been required. with resultant cost reduction.

It is another object to provide a cooling system that is not subject to freeze-ups. exterior frosting of lines. or excessive cooling or heating of the load.

It is also an object to provide a cooling system using a coolant that is non-corrosive. non-flammable. and not productive of deposits within the lines and components of the system; this permits the system to operate over long periods of time without maintenance.

Still another object is to provide a cooling system whose moving parts run continuously. without the need for regulating devices and consequently without wear or replacement of such devices. Additionally. it is an object to provide a cooling system that is simple in design and operation and has fewer moving parts than prior art devices. resulting in longer life. more dependable operation. and less expensive manufacture.

A liquid cooled thermal load particularly compatible with the system of the invention has a fairly large temperature difference between the input and output cooling fluid and functions best with a cold coolant. An example of a thermal load to which this system is particularly well suited is a liquid cooled. high vacuum diffusion pump for use in vacuum evaporators. electron microscopes and other high vacuum equipment. Such pumps usually have a liquid cooled trap to prevent back-diffusion of the pumping medium vapor into the working space. The cooling system of the present invention can furnish very cold fluid for vapor traps and therefore improve the cleanliness and pumping speeds of the vacuum systems.

In addition. because the input coolant can be colder than that provided by prior art cooling systems. the diffusion pump ean be run with slightly hotter coolant output. In practice. this makes it possible to increase the pumping speed of a liquid cooled diffusion pump.

While the cooling system of the present invention works especially well with loads of the type described. the system will be self-regulating to a large extent even with thermal loads of other sorts. and is not limited to use with loads having a large temperature differential between input and output coolant.

The viscosity regulated cooling system of the invention comprises a closed coolant circulating system adapted to contain a coolant whose viscosity varies inversely with its temperature; the circulating system has coolant circulating means for supplying and circulating coolant through the system. refrigeration means for cooling the coolant. load means for bringing the coolant. after it has been cooled by the refrigeration means. into heat absorbing relationship with the load device being cooled. and flow restriction means downstream of the load means and upstream of the circulating means for limiting flow of the coolant responsive to its viscosity.

In preferred embodiment. the cooling system further includes temperature normalizing means downstream of the flow restriction means for causing the temperature of the coolant to approach an ambient temperature after passng through the load means. and the coolant viscosity varies by at least a factor of 4 over the operating temperature range of the system. The operating temperature range is defined as the difference between the temperature of the coolant when input to the load means and the temperature of the coolant when output from the load means. For relatively small cooling systems using for example V4 OD to bring in the load means. the coolant preferably has a maximum viscosity of 500 centistokes at the temperature of the coolant when input to the load means. and a freezing point or glass point" at least 10F below the lowest possible temperature attainable in the heat exchange due to evaporation of refrigerant liquid in the evaporator.

Other objects. features and advantages will appear from the following description of a preferred embodiment of the invention. taken together with the attached drawings thereof. in which:

FIG. 1 is a schematic view of the cooling system of the invention. and

FIG. 2 shows the viscosity-temperature relationship for several possible coolants over a relevant range of temperatures.

Referring to the drawings and particularly to FIG. I. the cooling system 10 of the invention may be regarded generally as comprising two subsystems, a coolant circulating subsystem I2 and a refrigeration subsystem 14.

The refrigeration subsystem I4 is of a standard type and includes a compressor and condenser assembly I7. from which liquid refrigerant (such as Du Pont Freon I2 refrigerant) flows through capillary tubing 20. providing flow restriction. and thence to the evaporator 22; evaporated refrigerant returns to compressor/condenser assembly I7. Compressor/condenser assembly I7 is a continuously running, forced aircooled refrigeration assembly comprising compressor 16 and condenser 18. of the standard type found in refrigeration and freezer equipment. but without pressure switches. thermostats or other regulating devices. The thermal load capacity of the assembly is of the order of the thermal load to be placed on the cooling system of the invention. but not quite as great. As an example. in cooling a 400 watt load. a U5 hp compressor of standard field design has been successfully used in the system of the invention.

Coolant circulating subsystem I2 includes a coolant reservoir 34 and pump 24 which together comprise coolant circulating means. supplying coolant to the heat exchanger 26. Heat exchanger 26 together with the refrigeration subsystem I4 serve as refrigeration means for cooling the coolant. which then goes to load means 3]. which may be. for example. passageways or coils in thermal contact with the load device 28. In load means 3!. heat is absorbed by the coolant from load device 28, and the heated coolant flows to restrictor 30; from there the coolant goes to a temperature normalizing device in the form of intercooler 32. which brings the coolant to the ambient temperature before it is recirculated by the circulating means.

Referring now to FIG. 2. the fluid coolant to be used in coolant circulating subsystem 12 may be selected according to the individual requirements ofthe particular load to be cooled. but the primary requirement in all cases is that over the operating temperature range the viscosity of the coolant decreases rapidly as the coolant temperature rises. The operating temperature range of the system is defined as the difference between the temperature of the coolant when input to the load means and the temperature of the coolant when output from the load means. A viscosity ratio over the operating temperature range of 2:1 to 300:] is suitable; in the preferred embodiment herein described. the viscosity ratio is 4:l over a range of +14F to +l40F.

An additional requirement. dependent on the particular construction of the load means. is that the viscosity of the coolant at the lowest operating temperature should not be so great as to impede proper flow through the load means.

In a relatively small system using for example V4 OD to being in the load means the viscosity of the coolant at 40C should not be greater than 300-500 cs. for a Freon (R) 12 refrigerant system. (300 cs. is about the viscosity of 30 wt. motor oil at room temperature). For this reason motor oil. which has a viscosity of 20.000 cs. at 20F. is not suitable for use as a coolant. Further. the freezing point or "glass point of the coolant must be well below the lowest temperature to which it might be cooled in the particular application.

Further. though not essential to operation of the cooling system of the invention. in preferred embodiment. it is desirable to employ a coolant that is nontoxic and nonirritating. and has a high flash point. in case of accidental spillage. For example. isopropyl alcohol has been employed as a coolant. giving satisfac tory operation of the system. but it is not in practice desirable for use in cooling a load device such as a diffusion pump. which employs a heater, because of the fire hazard.

Finally. water based coolants or coolants mixed with water. even though otherwise usable, are undesirable because of the possiblity of rust or corrosion over long periods of operation. For example. brine. which has been commonly used in cooling systems. is corrosive. and may also freeze at temperatures above the lowest possible operating temperatures of the heat exchanger of the coolant subsystem. Similarly. although the particular system described herein will operate using a 50/50 mixture ofethylene glycol and water. such a mixture is also corrosive. and additionally has a freezing point of 34C'. since the coolant in the heat exchanger might be cooled to 40C in case of low ambient tern perature combined with a no-load condition or a coolant flow stoppage. this freezing point makes the mixture unsatisfactory in practice.

It has been found that several of the Dow Corning (RJ 200" silicone fluids are satisfactory coolants in the system of the invention. ln particular the 5. l and 20 centistokes fluids have been found to satisfy the requirements of the system. Higher viscosity fluids. such as the 100 cs. fluid. have too high a viscosity at the lower range of the operating temperatures to flow properly through most load means.

It is an additional advantage that the Dow 200" silicone fluids do not produce deposits in the lines and components ofthe system. and additionally function to lubricate the pump. which is thus self-lubricating. The

coolant system may therefore be sealed. and operates without loss or contamination of the coolant. Such a system may run as long as [0 or years without main tenanee.

In the particular embodiment described herein. the 50 centistoke silicone fluid has been employed; it has a flash point of 275F. while lower viscosity fluids have lower flash points. which is undesirable in case of spill age. This fluid has a viscosity ratio of about 7: 1 over the temperature range of 40F to +lF and its viscosity at 40F is cs.; its freezing point is l48F.

It will be appreciated that this coolant has been selected for the particular application of the preferred embodiment herein described; but the principle of the invention may be applied to cooling devices with differ' ent operating temperature ranges. for which other coolant fluids may be selected according to the criteria given above.

More in detail. referring again to P16. 1, evaporator 22 and heat exchanger 26 together form an evaporator- /exchanger assembly 27 where liquified refrigerant in system 14. received from compressor/condenser assembly 17. is evaporated in order to absorb heat from the circulating liquid coolant in system 12. The coolant passages in heat exchanger 26 are relatively large in order to permit good coolant flow at low temperatures despite increased viscosity of the coolant.

The coolant flows counter to the liquid refrigerant in exchanger assembly 27 in order to provide a maximun difference in temperture between the coolant and the refrigerant. Exchanger assembly 27 operates at maximum efficiency with this arrangement. which also ensures that the output coolant has the lowest possible temperature. Exchanger assembly 27 preferably provides two or more thermally separate sections to prevent internal thermal short-circuiting caused by metalto-metal contact between adjoining tubing coils or passages. whenever exchanger 27 is operating with a large difference in temperature between coolant input and output.

A constant pressure variable volume pump 24, which may be of the centrifugal type operating at very low loading. furnishes coolant at constant pressure to evaporator/exchanger 27. The partially filled. sealed coolant reservoir 34 provides reserve fluid. thermal buffering and space for thermal expansion of the coolant.

A fixed or adjustable restrictor 30, such as a needle valve or other flow restriction device. with a flow capacity that is small compared with the capacity of the coolant liquid lines and the capacity of coolant pump 24, is provided immediately downstream from the thermal load means 31.

A forced air-cooled coolant intercooler (heat exchanger] 32. exposed to the ambient air. takes up heat from the ambient air whenever the coolant temperature is below ambient temperature and radiates heat into the air whenever the coolant temperature is above ambient temperature. lntercooler 32 could alternatively be water-cooled by being exposed to a flow of water at ambient temperature. The intercooler must be large enough to provide a thermal load on refrigeration subsystem 14 when there is no external thermal load. as. for example. when the load device 28 to be cooled is not operating. Under such conditions. the coolant from load means 31 is warmed to be at or near the ambient temperature. which ensures that the coolant when input to heat exchanger 26 cannot be colder than some preselected temperature.

In a particular embodiment. the cooling system operated in an environment whose ambient temperature varied between 70and 85F. The load 28 was a diffusion pump on a vacuum evaporator; the load pump was heated by a 400-watt heater. The coolant was Dow 200 5 cs. silicone fluid. and the load means 3| was A inch o.d. copper tubing wrapped around the outside of the load pump. Compressor/condensor unit 16-18 was a 1/5 hp. unit made by Tecumseh Products Co. of Tecumseh. Michigan. The intercooler 32 was constructed of 4.5 turns of 5/l6 inch o.d. copper tubing. having a total length of l3 feet; the circulating pump 24 was a "Series 3M[)" magnetic drive pump manufactured by the Little Giant Corporation of Oklahoma City. Oklahoma. The reservoir was a copper container measuring approximately 3 inches in diameter X 6 inches long. In operation. the temperature of the coolant at the inlet end of the diffusion pump (load) was +14F. while the outlet temperature was about I30F. Restrictor 30 was a needle valve set to give a flow of I65 cc/min.

OPERATION The cooling system of the invention is viscosity regulated.

In routine operation. the viscosity of the coolant varies inversely and rapidly with its temperature over the operating temperature range. If the temperature of the coolant output from load 28 rises. coolant viscosity decreases. causing coolant flow through restrictor 30 to increase. The increased flow of coolant through the load removes more heat. and the load is returned to a lower temperature and vice versa.

The addition of intercooler 32 to the system improves the efficiency of its operation. In normal operation. the temperature of the coolant output from the load always exceeds the temperature of the ambient air; therefore intercooler 32 normally operates to assist refrigeration subsystem 14 by radiating heat from the coolant directly to the ambient air.

The refrigeration subsystem 14 operates continuously whenever the liquid cooling system of the invention is operating. Therefore. if thermal load 28 is removed or its heat output is low. there is a tendency for the temperature ofthe coolant output from the load to drop. However. under these conditions. intercooler 32 operated as a load on refrigeration subsystem 14, because intercooler 32 takes up heat from the ambient air whenever the coolant temperature is below ambient air temperature. The intercooler is designed to be large enough to provide a substantial load on the refrigeration subsystem. ensuring that the coolant temperature cannot fall below a preset minimum level.

Under the condition of no load. the viscosity of the coolant at the restrictor is very high. and therefore the flow of chilled coolant to the load is very small. As a result a non-operating load that is exposed to the ambient air to any great degree does not drop much below ambient air temperature.

If refrigeration system 14 fails during operation of the system. while the load is in operation and putting out heat. the heated coolant from the load is cooled in the intercooler. which radiates heat to the ambient air. The decrease in the coolant viscosity at the restrictor greatly increases the circulation of the coolant. These effects help prevent destructive overheating of the load. despite failure of the refrigeration system.

Because the compressor of the cooling system of the present invention runs continuously, and the system 5 starts up initially without liquifled compressed refrigerant in the evaporator/heat exchanger 27, overload of the compressor is avoided. thereby avoiding damage to the compressor motor. In addition. over load devices are normally used with such compressor motors. and are subject to wear and arcing; such devices are not needed in the present system.

The cooling system as a whole has fewer moving parts than many prior art cooling systems. and such moving parts as there are run continuously. rather than cycling on and off. This design reduces the initial cost of the system as well as maintenance and repair costs. In addition. this system operates dependably for long periods without maintenance.

What is claimed is:

l. A viscosity regulated cooling system for cooling a load device. comprising a closed coolant circulating system adapted to contain a coolant whose viscosity varies inversely with its temperature and having coolant circulating means for supplying and circulating coolant through said closed system. refrigeration means for cooling said coolant load means for bringing said coolant after being cooled by said refrigeration means into heat absorbing relationship with the load device. and flow restriction means downstream of said load means and upstream of said circulating means for limiting flow of said coolant responsive to its viscosity as established by said load means. and

35 termperature normalizing means connected downstream of said flow restriction means for causing the temperature of said coolant to approach an ambient temperature after passing through said load means.

2. A viscosity regulated cooling system for cooling a load device. comprising a closed coolant circulating system adapted to contain a coolant whose viscosity varies inversely with its temperature. and having coolant circulating means for supplying and circulating said coolant through said system.

refrigeration means for cooling said coolant.

load means for bringing said coolant after being cooled by said refrigeration means into heat absorbing relationship with the load device.

flow restriction means downstream of said load means and upstream of said circulating means for limiting flow of said coolant responsive to its viscosity as established by said loan means. and

temperature normalizing means downstream of said flow restriction means for causing the temperature of said coolant to approach an ambient temperature after passing through said load means.

said coolant viscosity varying by at least a factor of 4 over the operating termperature range of said system. said operating temperature range being defined as the difference between the temperature of said coolant when input to said load means and the temperature of i said coolant when output from said load means. and

said coolant having a maximum viscosity of 500 centistokes the temperature of said coolant when input to said load means. and a freezing point at least 10F below the temperature of said coolant when input to said load means.

3. A cooling system for cooling a load device comprising a closed coolant circulating system adapted to contain a coolant and having coolant circulating means for supplying and circulating coolant through said closed system refrigeration means for cooling said coolant. load means for bringing said coolant. after being cooled by said refrigeration means, into heat absorbing relationship with the load device. and temperature normalizing means downstream of said flow restriction means for causing the temperature of said coolant to approach an ambient temperature after passing through said load means. that improvement comprising a coolant whose viscosity varies inversely with its temperature. and flow restriction means downstream of said load means and upstream ofsaid circulating means for limiting flow of said coolant responsive to its viscosity as established by said load means. 4. In a cooling system for cooling a load device. comprising a closed coolant circulating system adapted to contain a coolant and having coolant circulating means for supplying and circulat ing coolant through said closed system.

refrigeration means for cooling said coolant. and

load means for bringing said coolant. after being cooled by said refrigeration means. into heat absorbing relationship with the load device.

that improvement comprising a coolant whose viscosity varies inversely with temperature by at least a factor of 4 over the operating temperature range of said system.

said operating temperature range being defined as the difference between the temperature of said coolant when input to said load means and the temperature of said coolant when output from said load means.

said coolant having a maximum viscosity of 500 centistokes at the temperature of said coolant when input to said load means. and a freezing point or glass point" at least l0F below the lowest possible temperature attainable in the heat exchanger.

flow restriction means downstream of said load means and upstream of said circulating means for limiting flow of said coolant responsive to its viscosity as established by said load means. and

temperature normalizing means connected down stream of said flow restriction means for causing the temperature ofsaid coolant to approach an ambient temperature after passing through said load means. 

1. A viscosity regulated cooling system for cooling a load device, comprising a closed coolant circulating system adapted to contain a coolant whose viscosity varies inversely with its temperature and having coolant circulating means for supplying and circulating coolant through said closed system, refrigeration means for cooling said coolant load means for bringing said coolant after being cooled by said refrigeration means into heat absorbing relationship with the load device, and flow restriction means downstream of said load means and upstream Of said circulating means for limiting flow of said coolant responsive to its viscosity as established by said load means, and termperature normalizing means connected downstream of said flow restriction means for causing the temperature of said coolant to approach an ambient temperature after passing through said load means.
 2. A viscosity regulated cooling system for cooling a load device, comprising a closed coolant circulating system adapted to contain a coolant whose viscosity varies inversely with its temperature, and having coolant circulating means for supplying and circulating said coolant through said system, refrigeration means for cooling said coolant, load means for bringing said coolant after being cooled by said refrigeration means into heat absorbing relationship with the load device, flow restriction means downstream of said load means and upstream of said circulating means for limiting flow of said coolant responsive to its viscosity as established by said loan means, and temperature normalizing means downstream of said flow restriction means for causing the temperature of said coolant to approach an ambient temperature after passing through said load means, said coolant viscosity varying by at least a factor of 4 over the operating termperature range of said system, said operating temperature range being defined as the difference between the temperature of said coolant when input to said load means and the temperature of said coolant when output from said load means, and said coolant having a maximum viscosity of 500 centistokes the temperature of said coolant when input to said load means, and a freezing point at least 10*F below the temperature of said coolant when input to said load means.
 3. A cooling system for cooling a load device comprising a closed coolant circulating system adapted to contain a coolant and having coolant circulating means for supplying and circulating coolant through said closed system refrigeration means for cooling said coolant, load means for bringing said coolant, after being cooled by said refrigeration means, into heat absorbing relationship with the load device, and temperature normalizing means downstream of said flow restriction means for causing the temperature of said coolant to approach an ambient temperature after passing through said load means, that improvement comprising a coolant whose viscosity varies inversely with its temperature, and flow restriction means downstream of said load means and upstream of said circulating means for limiting flow of said coolant responsive to its viscosity as established by said load means.
 4. In a cooling system for cooling a load device, comprising a closed coolant circulating system adapted to contain a coolant and having coolant circulating means for supplying and circulating coolant through said closed system, refrigeration means for cooling said coolant, and load means for bringing said coolant, after being cooled by said refrigeration means, into heat absorbing relationship with the load device, that improvement comprising a coolant whose viscosity varies inversely with temperature by at least a factor of 4 over the operating temperature range of said system, said operating temperature range being defined as the difference between the temperature of said coolant when input to said load means and the temperature of said coolant when output from said load means, said coolant having a maximum viscosity of 500 centistokes at the temperature of said coolant when input to said load means, and a freezing point or ''''glass point'''' at least 10*F below the lowest possible temperature attainable in the heat exchanger, flow restriction means downstream of said load means and upstream of said circulating means for limiting flow of said coolant responsive to its viscosity as established by said load means, and temperature normalizing means connected downstream of said flow restriction means for causing the temperature of said coolant to approach an ambient temperature after passing through said load means. 